Strong LP candidate from Florida makes some headway in libertarian leaning states and his home state.1890s Bryant-style populism makes its way into the westDemocratic candidate from Kentucky with VP from New Mexico.Republicans run someone from Connecticut with a VP from Wisconsin.

As for what would make it happen... err, the center of the country, with low population, becomes rich and prosperous, thus resulting in its Republican lean. Rich people leave the northeast for the west. The sunbelt is taken over by immigrants, and Minnesota was always pretty nuts anyway, and probably convinced neighboring Wisconsin and Michigan to join in on the fun. Ohio finally gets mad about free trade, and starts voting solidly Democrat.

The Democrat campaigns on using DC residents as slave labor in order to appease the poverty of the nation's crust. Alaskans get tired of the two major parties, and half of the population votes write-in.

I kind of stole Liberty and Gabu's maps and merged them together in a way:

The American West becomes tired of the east coast ruling everything wants to be come an independent nation in the name of states' rights(Republicans side with West). The American East wants to keep the Union as a whole in the name of economic growth (Democrats side with East). The Heartland is undecided and is worried about a war with them as the central ground (Libertarians gain support of some Centralites, which explains the >40% only there).